In the prior art, carbon brushes used for supplying power in a rotating electrical machine and the like are manufactured through pressure molding of raw graphite particles, metal particles, particles of solid lubricant such as molybdenum disulfide (MoS2), and organic binder, and calcining of the molded product. As schematically shown in FIG. 9A, a conventional carbon brush 90 has a configuration in which raw graphite particles 91, metal particles 92, and particles of solid lubricant 93 are dispersed. The raw graphite particles 91 are produced through, for example, a method in which graphitizing material is subjected to a high-temperature treatment to obtain graphitized material, and the graphitized material is then ground into powder, or a method in which graphitizing material is ground and then subjected to a high-temperature treatment to obtain graphitized material. The raw graphite particles 91 having various particle diameters can be produced depending on the degree of grinding. The raw graphite particles 91 having the average particle diameter of approximately 30 μm are used as raw material for the carbon brush 90 having reliable performance. Copper particles are used as the metal particles 92 in a suitable manner since the copper particles have high conductivity.
In the conventional carbon brush 90, carbide 94 of the organic binder coats the surface of each raw graphite particle 91. Furthermore, a number of the raw graphite particles 91 are adhered to one another to form graphite grains 95. The carbide 94 of the organic binder is depicted with thick black lines coating the outer circumferences of all the raw graphite particles 91 in the drawing. Each graphite grain 95 is configured by a number of the raw graphite particles 91 located in a region surrounded by a dashed line in the drawing. Furthermore, according to the carbon brush 90, binder (not shown) is applied to the outer surface of each graphite grain 95, thereby causing the graphite grains 95 to adhere to one another via the binder. In addition, the metal particles 92 and the solid lubricant particles 93 are dispersed in gaps among the graphite grains 95.
A metal graphite brush disclosed in, for example, Japanese Laid-Open Patent Publication No. 5-144534 is known as such a carbon brush 90. The metal graphite brush is formed through pressure molding of graphite powder and metallic powder, which are mixed with a predetermined compounding ratio, into a predetermined shape, and calcining of the molded product. Copper in the metal includes powder of microscopic particles and powder of large particles having 20 to 150 times the particle diameter of the powder of the microscopic particles. The compounding ratio of the powder of the microscopic particles to the powder of the large particles is 4:6 to 6:4. Since at least two types of blended powder, which are the powder of the microscopic particles and the powder of the large particles, are used as the metallic powder, which is one of the brush materials, the metal graphite brush suppresses sliding noise and self-excited vibration noise generated by frictional sliding between the metal graphite brush and a member along which the metal graphite brush slides. The metal graphite brush also has high specific resistance and long life (wear resistance).
According to the conventional carbon brush 90, however, the particle diameters of the graphite grains 95 vary in a wide range as shown in FIG. 9B. FIG. 9B shows a state where the metal particles 92 and the solid lubricant particles 93 are removed from the schematic diagram of FIG. 9A emphasizing the dispersed state of the graphite grains 95 and the raw graphite particles 91. The variation of the particle diameters of the graphite grains 95 causes large holes 96a and small holes 96b to exist simultaneously as shown by, for example, an electron microscopic picture of FIG. 3B. The holes 96a, 96b are spaces in which none of the raw graphite particles 91, the metal particles 92, the solid lubricant particles 93, the carbides 94 of the binder, and the graphite grains 95 exist.
The wide variation of the size and distribution of the holes 96a, 96b and the fact that the large holes 96a are easily formed leads to, for example, generation of a crack 97 as shown by the electron microscopic picture in FIG. 3D when a great load is applied to the carbon brush 90. This lowers the strength of the carbon brush 90. Furthermore, dispersing of the metal particles 92 tends to become uneven, which increases the possibility of various problems such as increasing voltage drop and frictional noise.